Compact lighting system and display device

ABSTRACT

A compact backlight system for illuminating a display device has a front wall and a rear wall situated opposite thereto. At least one light source comprising a light-emitting diode is provided with a translucent lens shaped cover. The system has at least one light input structure for coupling light from the light source into the light-emitting panel. During operation, light originating from the light source is incident on the light input structure and distributes itself in the light-emitting panel. According to the invention the light input structure is conically or frustoconically shaped towards the light source. The thickness d p  of the light-emitting panel is smaller than the diameter d c  of the translucent lens-shaped cover of the light source. Preferably, the light input structure is of prismatic or pyramidal shape.

The invention relates to a lighting system provided with alight-emitting panel comprising

-   -   a front wall, a rear wall situated opposite thereto,    -   at least one light source comprising a light-emitting diode        provided with a translucent lens-shaped cover,    -   at least one light input structure for coupling light from the        light source into the light-emitting panel,    -   wherein, during operation, light originating from the light        source is incident on the light input structure and distributes        itself in the light-emitting panel.

The invention also relates to a display device provided with saidlighting system.

Such lighting systems are known per se and are also referred to as edgelighting systems. They are used inter alia as backlighting systems in(picture) display devices, for example for TV sets and monitors. Suchlighting systems are particularly suitable for use as backlights fornon-emissive displays such as liquid crystal display devices, alsoreferred to as LCD panels, which are used in (portable) computers or(cordless) telephones.

Said display devices usually comprise a substrate provided with aregular pattern of pixels which are each controlled by at least oneelectrode. The display device utilizes a control circuit for achieving apicture or a datagraphical display in a relevant field of a (picture)screen of the (picture) display device. The light originating from thebacklight in an LCD device is modulated by means of a switch ormodulator, various types of liquid crystal effects being used. Inaddition, the display may be based on electrophoretic orelectromechanical effects.

Such lighting systems are also used as luminaires for general lightingpurposes or for shop lighting, for example shop window lighting orlighting of (transparent or semi-transparent) plates of glass or of(transparent) plates of synthetic resin on which items, for examplejewelry, are displayed. Such lighting systems are further used as windowpanes, for example for causing a glass wall to radiate light undercertain conditions, or to reduce or block out the view through thewindow by means of light. A further alternative application is the useof such lighting systems for illuminating advertising boards.

In the lighting systems mentioned in the opening paragraph, the lightsource used usually comprises a plurality of optoelectronic elements,also referred to as electro-optical elements, for exampleelectroluminescent elements, such as light-emitting diodes (LEDs). Theselight sources are usually provided in the vicinity of or tangent to alight-transmitting edge surface of the light-emitting panel, in whichcase light originating from the light source is incident on thelight-transmitting edge surface during operation and distributes itselfin the panel.

A lighting system for illuminating an LCD panel is known from U.S. Pat.No. 5,921,652. The light source used is formed by light-emitting diodes(LEDs) which couple light into a light-emitting panel, also referred toas light pipe, via a so-called light transition area. The light is mixedin said light transition area.

A lighting system of the above type occupies too much space.

It is an object of the invention to wholly or partly eliminate the abovedisadvantage. According to the invention, a lighting system of the kindmentioned in the opening paragraph is for this purpose characterized

-   -   in that the light input structure is conically or        frustoconically shaped towards the light source, and    -   in that the thickness d_(p) of the light-emitting panel is        smaller than the diameter d_(c) of the translucent lens-shaped        cover of the light source.

By increasing the area of the light input structure, the efficiency withwhich light originating from the light source is coupled into thelight-emitting panel is increased. A light source based on alight-emitting diode normally comprises a light-emitting part (a chip)covered by a translucent lens-shaped cover. The dimensions of thislens-shaped cover as well as the shape of the light input structurelargely determine how (efficiently) light is transferred from the lightsource to the light-emitting panel.

In the known lighting system, the light input structure is an edgesurface, generally a side wall of the light-emitting panel. Said edgesurface is provided between the front wall and the rear wall of thelight-emitting panel. In order to sufficiently efficiently couple lightoriginating from the light source into the light-emitting panel of theknown lighting panel with edge surfaces, the thickness of thelight-emitting panel has to be larger than the characteristic dimensionof the translucent lens-shaped cover, for example the diameter d_(c) ofthe translucent lens-shaped cover.

The tapered shape of the light input structure which gradually narrowsin the direction of the light source according to the invention enablesthe light from the light source to be coupled into the light-emittingpanel with a relatively high efficiency. Without being obliged to giveany theoretical explanation, there exists an inverse relationshipbetween the thickness of the light-emitting panel and the efficiencywith which light from the light source is coupled into thelight-emitting panel. Due to the relatively large effective surface ofthe light input structure, the dimensions of the light-emitting panelcan be reduced without the relatively high efficiency with which thelight is coupled into the light-emitting panel being adversely affected.A compact lighting system is obtained through the measure according tothe invention.

Preferably, the light input structure is of prismatic or pyramidalshape. Light input structures in the form of a prism or a pyramid areparticularly useful in attaining a large specific surface of the lightinput structure. The apex (or the frustoconical end) of the pyramid ispreferably directed towards the (geometrical) center of the translucentlens-shaped cover and is preferably (approximately) in contact with thetranslucent lens-shaped cover.

In a preferred embodiment of the lighting system according to theinvention the light input structure is an integral part of thelight-emitting panel. Preferably, the light-emitting panel and the lightinput structure of the lighting system are molded in one piece.

A preferred embodiment of the lighting system according to the inventionis characterized in that light-guidance means are provided in thevicinity of the light source and the light-emitting panel so as toencompass the light input structure, said light-guidance means guidingthe light originating from the light source towards the light inputstructure.

Preferably, the (inner) surface of the light-guidance means facing thelight input structure is reflective or provided with a reflectivematerial. The light-guidance means function as a (specular) mirrorguiding the light originating from the light source towards the lightinput structure.

Preferably, the light-guidance means are elliptically shaped.Elliptically shaped mirrors efficiently guide the light originating fromthe light source towards the light input structure. In an alternativeembodiment the light-guidance means (9) are faceted. Faceting renders itpossible to direct the light in specific directions during reflection.

The light source used may be formed by one or more light-emitting diodes(LEDs) and/or by LEDs of different colors which are combined with oneanother. Colors may be mixed in a desired manner through a suitable useof LEDs, for example for generating white light of a desired colortemperature. Preferably, the light source comprises three light-emittingdiodes. The LEDs preferably comprise combinations of red, green, andblue LEDs known per se, or, for example, combinations of red, green,blue, and amber LEDs. LEDs with three light emission wavelengths mayalso be realized by means of two LEDs with different light emissionwavelengths, wherein the LEDs of one of the types are (partly) providedwith a phosphor, such that the light emission of the LED is converted bythe phosphor into light of a third, desired light emission wavelength. Acombination, known per se, of the red, green, and blue LEDs renders itpossible to realize color changes independently of the status of thedisplay device. The use of LEDs has the further advantage that dynamiclighting possibilities are obtained. For this purpose, a sensor presentat one of the edge surfaces measures the optical properties of the lightemitted by the light source during operation.

The quantity of light emitted by the LEDs is adjusted in that theluminous fluxes of the light-emitting diodes are varied. This control ofthe luminous flux usually takes place in an energy-efficient manner.Thus the LEDs can be dimmed without an appreciable loss of efficacy.Preferably, the intensity of the light emitted by the light-emittingdiodes is variable in response to the illumination level of a picture tobe displayed by the display device or in response to the level of theambient light. Preferably, the color point of a picture displayed by thedisplay device is determined by the lighting system. An (improved)dynamic range (for example contrast) of the picture to be displayed bythe display device is achieved thereby.

Preferably, each of the light-emitting diodes has a luminous flux of atleast 10 lm. LEDs with such a high output are also referred to as LEDpower packages. The use of these high-efficiency, high-output LEDs hasthe specific advantage that the number of LEDs required for a desired,comparatively high light output can be comparatively small. Thisbenefits the compact construction and the efficiency of the lightingsystem to be manufactured. Further advantages of the use of LEDs are acomparatively very long service life, the comparatively low energyconsumption, and the low maintenance costs for a lighting system withLEDs.

The invention will now be explained in more detail with reference to anumber of embodiments and a drawing, in which:

FIG. 1 is a cross-sectional view of a display device comprising anembodiment of the lighting system according to the invention, and

FIG. 2 shows the light input efficiency of various light inputstructures and various types of light-guidance means as a function ofthe thickness of the light-emitting panel.

The Figures are purely diagrammatic and not drawn true to scale. Somedimensions are particularly strongly exaggerated for reasons of clarity.Equivalent components have been given the same reference numerals in theFigures whenever possible.

FIG. 1 is a cross-sectional view of a display device comprising anembodiment of the lighting system according to the invention. Thelighting system comprises a light-emitting panel 1 of alight-transmitting material. The panel 1 is manufactured, for example,from a synthetic resin, from acryl, from polycarbonate, from pmma, forexample Perspex, or from glass. Light is transported through thelight-emitting panel 1 during operation, utilizing total internalreflection (TIR). The light-emitting panel has a front wall 2 and a rearwall 3 opposite thereto. In FIG. 1 at least one light source 6 isprovided comprising a light-emitting diode 7 with a translucentlens-shaped cover 8. The lighting system comprises at least one lightinput structure 4 for coupling light from the light source 6 into thelight-emitting panel 1. During operation, light originating from thelight source 6 is incident on the light input structure 4 anddistributes itself in the light-emitting panel 1. According to theinvention, the light input structure 4 is conically or frustoconicallyshaped towards the light source 6.

By increasing the area of the light input structure 4, the efficiencywith which light originating from the light source 6 is coupled into thelight-emitting panel 1 is increased. The light input structure 4gradually narrows in the direction of the light source, enabling lightfrom the light source 6 to be coupled into the light-emitting panel 1with a relatively high efficiency. Due to the relatively large effectivesurface of the light input structure 4, the dimensions of thelight-emitting panel 1 are reduced. According to the invention, thethickness d_(p) of the light-emitting panel 1 is smaller than thediameter d_(c) of the translucent lens-shaped cover 8 of the lightsource 6 (see FIG. 1). A compact lighting system is obtained through themeasure according to the invention.

In FIG. 1 the light input structure 4 is separate from thelight-emitting panel 1. In an alternative embodiment of the lightingsystem, the light input structure is integral with the light-emittingpanel.

The light-emitting panel 1 emits light in the direction of the displaydevice during operation, for example a liquid crystal display (LCD)device 12. The assembly of the light source 6, the light-emitting panel1, and the LCD device 12, whether or not accommodated in a housing (notshown in FIG. 1), forms a display device for displaying, for example,(video) images.

The light-emitting panel 1 may further be provided with a sensor (notshown in FIG. 1) for measuring the optical properties of the light. Thissensor is coupled to control electronics (not shown in FIG. 1) forsuitably adapting the luminous flux of the light source 6. A feedbackmechanism can be realized by means of the sensor and the controlelectronics for influencing the quality and quantity of the lightcoupled out of the light-emitting panel 1.

FIG. 2 shows the light input efficiency of various light inputstructures and various types of light-guidance means as a function ofthe thickness d_(p) of the light-emitting panel. The efficiency isindicated by measuring the flux F of the light in the light-emittingpanel 1 relative to (in FIG. 2 as a percentage) the light emitted by thelight source 6. The diameter d_(c) of the translucent lens-shaped cover8 of the LED light source for the measurements in FIG. 2 is 5.5 mm. Thetriangles in FIG. 2 correspond to measurements carried out on alight-emitting panel wherein the light input structure is a flat edgesurface and the light-guidance means have flat surfaces. It can be seenfrom FIG. 2 (triangles) that the thickness d_(p) of the light-emittingpanel has to be relatively large, typically equal to or larger than thediameter d_(c) of the translucent lens-shaped cover of the LED lightsource, to obtain a flux efficiency higher than 80%. The squares in FIG.2 correspond to measurements carried out on a light-emitting panelwherein the light input structure is a flat edge surface and thelight-guidance means have elliptical surfaces. It can be seen from FIG.2 (squares) that the situation has improved due to theelliptically-curved surfaces of the light-guidance means. In thissituation the thickness d_(p) of the light-emitting panel has to berelatively large to obtain a flux efficiency higher than 80%. Thecircles in FIG. 2 correspond to measurements for a lighting systemaccording to an embodiment of the invention, employing a light-emittingpanel 1 wherein the light input structure 4 has a prismatic shapepointing towards the translucent lens-shaped cover 8 and thelight-guidance means 9 have elliptical surfaces (see FIG. 1). It can beseen from FIG. 2 (circles) that a relatively high flux efficiency(approximately 80%) is obtained for a thickness dp of the light-emittingpanel which is approximately half the diameter d_(c) of the translucentlens-shaped cover of the LED light source. A compact lighting system isobtained through the measure according to the invention.

It will be obvious that many modifications are possible to those skilledin the art within the scope of the invention.

The scope of protection of the invention is not limited to theembodiments given. The invention resides in each novel characteristicand each combination of characteristics. Reference numerals in theclaims do not limit the scope of protection thereof. The use of the verb“comprise” and its conjugations does not exclude the presence ofelements other than those specified in the claims. The use of theindefinite article “a” or “an” preceding an element does not exclude thepresence of a plurality of such elements.

1. A lighting system provided with a light-emitting panel comprising afront wall, a rear wall situated opposite thereto, at least one lightsource comprising a light-emitting diode provided with a translucentlens-shaped cover, at least one light input structure for coupling lightfrom the light source into the light-emitting panel, wherein, duringoperation, light originating from the light source is incident on thelight input structure and distributes itself in the light-emitting panelthe light input structure is conically or frustoconically shaped towardsthe light source, and the thickness d_(p) of the light-emitting panel issmaller than the diameter d_(c) of the translucent lens-shaped cover ofthe light source.
 2. A lighting system as claimed in claim 1, whereinthe light input structure is of prismatic or pyramidal shape.
 3. Alighting system as claimed in claim 1, wherein the light input structureis an integral part of the light-emitting panel.
 4. A lighting system asclaimed in claim 1, wherein light-guidance means are provided in thevicinity of the light source and the light-emitting panel so as toencompass the light input structure, said light-guidance means guidingthe light originating from the light source towards the light inputstructure.
 5. A lighting system as claimed in claim 4, wherein a surfaceof the light-guidance means facing the light input structure isreflective or provided with a reflective material.
 6. A lighting systemas claimed in claim 4, wherein the light-guidance means are ellipticallyshaped.
 7. A lighting system as claimed in claim 4, wherein thelight-guidance means are faceted.
 8. A lighting system as claimed inclaim 1, wherein the light-emitting diode has a luminous flux of atleast 10 lm.
 9. A display device provided with a lighting system asclaimed in claim
 1. 10. A display device as claimed in claim 9, whichdisplay device comprises a liquid crystal display.